Abstract. This paper describes an executable formal semantics of C expressed using a formalism based on term rewriting. Being executable, the semantics has been thoroughly tested against the GCC torture test suite and successfully passes over 96\% of 715 test programs. It is the most complete and thoroughly tested formal definition of C to date. The semantics yields an interpreter, debugger, and state space search tool "for free". The semantics is shown capable of automatically finding program errors, both statically and at runtime. It is also used to enumerate nondeterministic behavior. These techniques together allow the tool to identify undefined programs.